The present invention relates to a method for manufacturing a semiconductor device from a step of thinning a semiconductor device substrate to a step of dicing it, and a semiconductor device.
Recently, in the field of a semiconductor integrated circuit, developments in making a device more dense and more integrated are progressing and, as a mobile communicating device is getting smaller and lighter, the device is getting finer.
Generally, as an area of a device is getting smaller, the thermal resistance of the device is getting larger. In order to realize a device with a high performance and a high reliability, it is essential to reduce the thermal resistance of the device. Therefore, a variety of approaches have been taken to improve the effect of heat radiation.
The most effective way to reduce the thermal resistance of a device is to thin a whole substrate after an integrated circuit device pattern is formed on the substrate. However, since a GaAs substrate generally used is bristle and is breakable, when a GaAs substrate having a diameter of 3 inches or more is thinned to, for example, 50 xcexcm or less, a problem occurs that the thinned substrate is broken in subsequent steps of handling, transporting, mounting, and the like.
Generally, in order to prevent a semiconductor substrate from breaking when it is thinned, the substrate is fixed to a supporting plate. A photosensitive adhesive tape having an UV-sensitive adhesive layer is used to fix the semiconductor substrate. Since the UV-sensitive adhesive layer of the UV-sensitive adhesive tape is deteriorated in the adhesion strength by UV irradiation, it becomes easier to remove the thinned substrate from the supporting plate. Further, since no residue remains on the device side of the semiconductor substrate after removing, an UV-sensitive adhesive tape is extensively used.
For example, JP-A 216092/1994 describes a method for fixing a GaAs substrate on a supporting plate by using a tape with a composite structure consisting of a UV-sensitive adhesive layer and a heat foamable adhesive layer. In this method, a semiconductor substrate is fixed on a supporting plate by bonding the UV-sensitive adhesive layer to a surface of the semiconductor substrate on which a device is formed, and by bonding the heat foam able adhesive layer to the supporting plate. After thinning, the adhesion strength of the UV-sensitive adhesion layer is reduced by UV irradiation to remove the thinned semiconductor substrate.
Therefore, in this method, since a thinned substrate is handled in steps of dicing, mounting and the like, a problem remains that the thinned substrate is easy to be broken in handling in these steps.
In addition, JP-A 213662/1997 describes, as a method of preventing a substrate from breaking in the step of thinning the substrate, a method which comprises, first, forming a dicing groove having a predetermined depth on a wafer on which semiconductor devices are formed, fixing the wafer by adhering an adhesion sheet to the surface of the wafer on which semiconductor devices are formed and, then, grinding the back side of the wafer until reaching the groove to split into individual chips.
This method is effective for preventing the breakdown of thinned substrates because devices are split along dicing lines in a step of thinning. However, since the devices are perfectly split after thinning, all the devices can not be electrically connected in a step of back-metal plating to reinforce the substrate and, consequently, it is impossible to plate the whole surface of the wafer simultaneously. Thus, this method exhibits a very poor productivity.
Accordingly, an object of the present invention is to provide a method for manufacturing a device, which comprises a series of processes from a step of thinning a semiconductor device substrate having a large diameter, a step of reinforcing the thinned substrate, and a step of mounting the thinned device. More specifically, an object of the present invention is to provide a high-productive method of manufacturing a semiconductor device without damaging and polluting the semiconductor device substrate having a large diameter.
The method for manufacturing a semiconductor device substrate of the present invention is characterized by carrying out a series of processes from a step of uniformly thinning a whole surface of a semiconductor device substrate to a step of back-metal plating, while the semiconductor device substrate is unified with a dummy substrate. Therefore, the present invention allows a semiconductor device to be manufactured without breaking the thinned semiconductor device substrate even after the thinned substrate is separated from the dummy substrate.
First, the whole process of the present method for manufacturing a semiconductor device will be explained hereinafter.
A semiconductor device is bonded to a dummy substrate by using an adhesive sheet and the like to unify them. After the unified substrates are fixed on a pedestal support with wax and the like, the semiconductor device substrate is thinned by mirror-polishing. The unified substrates are fixed by vacuuming, and the semiconductor device substrate is thinned by grinding or by etching the backside of the semiconductor device substrate. After thinning, a step of back-metal plating is carried out in the unified state of the thinned semiconductor device substrate/the dummy substrate. Subsequently, the thinned substrate that has been plated is separated from the dummy substrate. After separation, the thinned substrate that has been plated is diced.
Particularly, in the case of polishing or grinding, an intrasubstrate fluctuation in the thickness of the dummy substrate to which the semiconductor device substrate is bonded with the adhesive sheet, is finally reflected in an intrasubstrate fluctuation in the thickness of the thinned GaAs semiconductor device substrate. Therefore, it is especially required that the intrasubstrate fluctuation in the thickness of the dummy substrate is small.
For example, when it is desired that a semiconductor substrate is thinned to 30 xcexcm, if the fluctuation within a surface in the thickness of the dummy substrate is 10 xcexcm, the thickness of the thinned semiconductor substrate becomes 20xcx9c40 xcexcm (30xc2x110 xcexcm), resulting in that the thickness of the substrate within the surface varies twice at maximum. Considering that the thermal resistance of a semiconductor substrate is approximately proportional to the thickness of the substrate, the thermal resistance of the substrate within the surface varies twice. That is, the fluctuation in the thickness of the dummy substrate results in the fluctuation in the thermal resistance of the device obtained after thinning and, in turn, it affects the reliability of the device.
Therefore, in the case of thinning a semiconductor device substrate to a thickness of 40 xcexcm or less, it is desirable that the intrasubstrate fluctuation in the thickness of the dummy substrate is within xc2x15 xcexcm.
Generally, when a substrate is thinned, in order to improve workability of thinning, an abrasive having a large particle size (e.g., particle size 9 xcexcm) is used to grind the substrate to achieve a predetermined thickness in a short period. The maximum difference in the thickness within the uneven surface that has been thinned is 2 xcexcm or more.
As will be explained later, the present inventors have found that when an Au plate layer was formed on a GaAs substrate, the maximum difference in the thickness within the uneven surface greatly affected an adherability between the GaAs substrate and the Au plate layer.
Generally, a back metal layer is formed on a substrate by dip-etching the substrate to clean its surface, depositing a power feed layer for plating on the surface and, thereafter, plating a metal. However, dip etching alone can not make the maximum difference in the thickness within the uneven surface below 0.5 xcexcm, and the adherability of the plate layer is insufficient. In order to improve the adhering strength between the GaAs substrate and the Au plate layer, it is essential that the surface of the substrate that has been thinned is mirror-polished and, further, a surface oxide layer is thoroughly removed by etching.
Thus, in the present invention, in order to improve the adherability, after thinning the GaAs substrate, the surface is mirror-polished by using an alumina abrasion having a particle size of 0.05 xcexcm or less and, then, the surface oxide layer is thoroughly removed with a phosphoric acid etchant so that the maximum difference in the thickness within the mirror-polished surface is suppressed below 0.2 xcexcm.
Additionally, from a viewpoint of the fluctuation in the thickness within the etched surface and controllability of the thickness, an appropriate range of etching depth is 0.5xcx9c30 xcexcm.
According to the procedure of the present invention, since steps of thinning and plating are carried out while a semiconductor device substrate is unified with a dummy substrate, the breakdown of the substrate does not occur. In addition, after separating the thinned substrate from the dummy substrate, since the thinned substrate is reinforced with the back metal, the thinned substrate can be handled without breaking in the step of separating and the final step of dicing the thinned substrate.
Further, the present invention provides a method for manufacturing a semiconductor device, which comprises, a step of thinning a semiconductor device substrate, fixing the semiconductor substrate to a dummy substrate with a heat foamable adhesive sheet, and fixing the dummy substrate to a pedestal support to thin the semiconductor substrate.
By using the heat foamable adhesive sheet as a sheet for fixing the semiconductor device substrate to the dummy substrate, the semiconductor device substrate is easily peeled off from the dummy substrate after thinning, and it can endure heat-up in the steps of depositing the power feeding metal and back-metal plating.
In addition, the present invention provides a method for manufacturing a semiconductor device, wherein the semiconductor device substrate is fixed to the dummy substrate by bonding a heat foamable-adhesive side of the heat foamable adhesive sheet to a device side of the semiconductor device substrate, and by bonding a normal adhesive side of the heat foamable adhesive sheet to the dummy substrate.
Since the adhesion strength of the heat foamable adhesive becomes almost zero by heating, the semiconductor device substrate is easily peeled off from the dummy substrate. In addition, since a heat foamable adhesive sheet is used in place of a conventional UV-sensitive adhesive sheet, a conventional exposure process is made redundant, and it is unnecessary for the dummy substrate to be transparent. Consequently, there is an advantage that the limit in selecting a substrate is abolished.
Further, the method for manufacturing a semiconductor device according to the present invention is characterized by that a temperature for heat-foaming the heat foamable adhesive sheet is higher than a temperature of the substrate during the step of back-metal plating.
In a step of back-metal plating, when a temperature of the substrate is elevated above the temperature for heat-foaming the heat foamable adhesive sheet in depositing a power feeding metal layer and plating a metal, the thinned semiconductor device substrate is separated from the dummy substrate and breaks. Even when heat-up occurs locally, the heat foamable adhesive sheet foams locally and, therefore, the thinned substrate breaks locally. This problem can be solved by using a heat foamable adhesive sheet having a higher foaming temperature than the temperature of the substrate in the step of back-metal plating.
Sill further, the present invention provides a method for manufacturing a semiconductor device, wherein the maximum difference in the thickness of the uneven surface of the substrate which has been thinned is 0.2 xcexcm or less, and wherein the thinned surface is treated by etching.
Generally, after grinding a substrate, the maximum difference in thickness within the ground surface is more than 1xcx9c2 xcexcm. Although the surface is cleaned by dip etching prior to plating, it is impossible to make the maximum difference in the thickness within the uneven surface below 0.5 xcexcm only by a dip etching. The maximum difference in the thickness within the uneven surface greatly affects the adherability between a GaAs substrate and an Au plate layer.
In the method of the present invention, in order to improve the adherability between the GaAs substrate and the Au plate layer, after thinning the substrate, the surface is mirror-polished and, then, the surface oxide layer is thoroughly removed so that the maximum difference in the thickness within the mirror-polished surface is suppressed below 0.2 xcexcm, and, subsequently, a power feeding layer is deposited prior to metal plating.
The present invention provides a method for manufacturing a semiconductor device, wherein the step of back-metal plating the semiconductor device substrate comprises adhering a metal for power feeding and, then, plating a metal thereon while the semiconductor device substrate is fixed to the dummy substrate with a heat foamable adhesive sheet.
According to the present invention, the whole surface of the thinned semiconductor device substrate can be plated with a back metal in a state where the semiconductor device substrate is fixed to the dummy substrate with a heat foamable adhesive sheet. A heat foamable adhesive sheet has a disadvantage that a part of the foamed adhesive remains on the substrate surface after removal of the substrate. However, since the thinned substrate is reinforced by a back metal with its whole surface plated, after the thinned substrate is peeled off from the dummy substrate, the remaining adhesive is thoroughly removed off by washing and ashing the substrate surface without breaking.
In addition, the present invention provides a method for manufacturing a semiconductor device, wherein the step of dicing the semiconductor device substrate comprises bonding a plated surface of the semiconductor device substrate to a surface to be foamed of the heat foamable adhesive sheet, bonding a dicing sheet to the opposite surface of the heat foamable adhesive sheet and, then, dicing them halfway in the dicing sheet.
When the thinned semiconductor device substrate is directly attached to a dicing sheet, devices break upon peeling the devices off from the dicing sheet due to the strong adhesion of the dicing sheet. In particular, when the thinned semiconductor device substrate is not so thick and the reinforcement with the back metal layer is not so strong, for example, in the case where the thickness of a device substrate 5 is 50 xcexcm or less, and the thickness of the Au plate layers 7, 8 is 5 xcexcm or less, when the device substrate 5 is directly attached to a dicing sheet 10, upon peeling devices off from the dicing sheet 10 in a step of mounting after dicing, the device substrate 5 is broken due to the strong adhesion of the dicing sheet 10.
Accordingly, in the present invention, since a heat foamable adhesive sheet capable of being easily peeled off by heating is attached to the device substrate and, then, a dicing sheet is attached to the heat foamable adhesive sheet, the devices are peeled off from the dicing sheet after dicing while the device is unified with the heat foamable adhesive, and, therefore, the device is not broken so much so that it is readily to handle the device.
Further, the present invention provides a method for manufacturing a semiconductor device, wherein the diced, thinned semiconductor device substrate/heat formable adhesive sheet is peeled off from the dicing sheet substrate and, then, the semiconductor device substrate is separate from the heat foamable adhesive sheet by heating.
When the thickness of the thinned substrate is 100 xcexcm or more, it is unnecessary to form a back metal because the thinned substrate does not break. In this case, the thinned device and the heat foamable adhesive sheet may be separated each other by heating the whole of the thinned, unified device/heat foamable adhesive sheet after dicing. Alternatively, only needed devices may be taken out and separated from the heat foamable adhesive sheet by heating.
A compound semiconductor GaAs substrate generally used as a semiconductor substrate is brittle and, particularly for a large substrate having a diameter of 2 inches or more, it is essential to prevent the substrate from breaking by handling.
According to the present invention, the semiconductor substrate is bonded to the dummy substrate to form a unified structure and, the semiconductor substrate can be handled from a step of thinning to a step of back-metal plating without breaking the semiconductor substrate. Since even a compound semiconductor substrate more unbreakable than a GaAs substrate, such as InP, GaN, SiC and the like, becomes breakable when the thickness becomes thin, the method of the present invention exhibits an effect to improve the handling property in a step of manufacturing a semiconductor device using a compound semiconductor substrate. That is, the method of the present invention is useful for manufacturing a semiconductor device in which the semiconductor device substrate is a compound semiconductor substrate.
Since a dummy substrate is required to be a substrate which is thin and unbreakable, and which has a uniform thickness, the present invention uses a substrate such as a silicone wafer, a glass plate, a ceramic plate, a metal plate and the like. A silicone wafer is particularly preferred because it is difficult to break and is excellent in uniformity of a thickness.
In addition, the GaAs semiconductor substrate manufactured by the method of the present invention is characterized by that the device substrate and the metal plate layer are diced vertically to the device surface of the substrate.
According to the present invention, since, in the step of thinning, the adherability between the GaAs semiconductor substrate and the back metal is improved by reducing the unevenness of the thinned substrate surface, and by mirror-polishing the thinned surface by etching, the GaAs semiconductor substrate and the back metal layer never be separated even when the both members are simultaneously diced vertically to the device surface.
Simultaneous and vertical dicing results in improvement of the productivity, and also has an effect that chip mounting can be done without damaging the chip because it is easy to hold an edge surface of the chip with a collet. Especially when a chip having a thickness of 20xcx9c50 xcexcm is handled, it is important that an edge surface is vertical.
In addition, according to the present invention, it is possible to reduce a thickness of the GaAs semiconductor device substrate to minimal about 20 xcexcm by the controllability of mirror-polishing and chemical etching, and to minimal about 100 xcexcm by the controllability of grinding.
Although a thermal resistance of a GaAs semiconductor device substrate is getting lower as the substrate is getting thinner, it is desirable that the thickness of the device substrate is 20 xcexcm or more in view of the flatness of a wiring on the surface of a GaAs semiconductor device and mounting of a semiconductor device. Although when the thickness of the device substrate becomes 100 xcexcm or more, it can be handled normally, the method of manufacturing of the present invention can also be applied to a device substrate having a thickness of 100 xcexcm or more. Additionally, a thermal resistance of a substrate is approximated by ln(2t/a) wherein xe2x80x9ctxe2x80x9d is the thickness of the substrate, and xe2x80x9caxe2x80x9d is the size of the substrate. Generally, in the case where the size of the substrate is from a few microns to a few ten microns, since the thermal resistance saturates when the thickness of the substrate becomes about 100 xcexcm or more, a further reduction in the thermal resistance can not be expected. Therefore, the preferred thickness of the device substrate is 20xcx9c100 xcexcm.
From a viewpoint of reinforcing a thinned substrate, it is desirable that the thickness of a plate layer is 5 xcexcm or more. On the other hand, when the thickness of the plate layer becomes 40 xcexcm or more, a production cost increases and it becomes more difficult to dice the metal plate layer for separating a device, resulting in a poor productivity.
In addition, the use of a semiconductor device of the present invention enables a radio set with a high reliable performance to be manufactured.
Since the thermal resistance of the thinned semiconductor device of the present invention is greatly decreased, there is an advantage that the use of the present device makes a portable terminal unit of a radio set high efficient, high reliable and light by multiple mounting.